Excimers are unstable, excited-state molecular complexes that occur only under extreme conditions, such as those temporarily existing in special types of gas discharge. Excimer complexes dissociate in less than a microsecond, releasing their binding energy in the form of ultraviolet radiation. The advantage of the excimer radiation as a light source for UV curing of compositions is that it provides radiation in a narrow band that can be matched to the absorbing radiation of a photoinitiator in the curing composition. In contrast, the mercury vapor lamps that often used for UV curing lithographic inks emit a broad band of ultraviolet, visible, and infrared radiation. Broadband radiation sources are problematic for plastic substrates because they tend to cause film deformation by heating the substrate.
Because excimer radiation delivers a narrow band of radiation, it is especially critical for the UV curing composition to be able to absorb a sufficient amount of energy at that wavelength to provide adequate cure. Nohr et al. in U.S. Pat. No. 6,265,458 describe photoinitiator selection for excimer curing. The pigment in inks interferes with light transmission, however, so that it is not only a matter of selecting an appropriate photoinitiator. It is generally thought that thinner films result in higher oxygen inhibition and a resultant decrease in the rate of cure, especially for dark colors.
To date, dark lithographic inks have been unable to provide print films sufficiently cured by excimer lamp and having the scratch resistance, overprintability, and adhesion to plastic substrates required of a commercially viable product, even though excimer lamps present a more energy efficient source of actinic radiation.